The present invention relates to a plasma processing method and an apparatus thereof for use in manufacturing electron devices and micro machines made of semiconductors.
In recent years, a thin film processing technique using plasma processing has become more and more important in the field of manufacturing semiconductor electron devices and micro machines.
As one example of prior art plasma processing methods, plasma processing with use of an inductively coupled plasma source will be described hereinbelow with reference to FIG. 8. In FIG. 8, a specified gas is introduced from a gas supply device 2 into a vacuum chamber 1 while being exhausted therefrom by a pump 3 serving as an exhauster to keep the vacuum chamber 1 within a specified pressure. Under such a condition, high-frequency power of 13.56 MHz can be supplied by a high-frequency power source 4 to a coil 23 to generate plasma in the vacuum chamber 1 to perform plasma processing of a substrate 7 mounted on a substrate electrode 6. In addition, there is provided a high-frequency power source for substrate electrode 8 for supplying high-frequency power to the substrate electrode 6, which enables control of ion energy reaching the substrate 7. It is noted that the coil 23 is disposed on top of a dielectric window 24. The gas is introduced into the vacuum chamber 1 through a plurality of gas supply holes 25 provided on a metal ring 16 which constitutes part of a side wall of the vacuum chamber 1.
However, in order to improve fine processability and enlarge the processing area, the flow of gas to be used in processing should be increased, and processing should be performed under lower pressure. This tends to induce abnormal electrical discharge called hollow cathode discharge in gas supply holes 25 in the prior art plasma processing.
A description of the hollow cathode discharge is as follows. In general, the surface of a solid in contact with plasma is negatively electrified due to a difference in thermal velocity between an electron and an ion, so that the solid surface obtains direct electric fields which send away electrons from the solid surface. In a space surrounded with the solid surface, like the inside of the gas supply hole 25 shown in the prior art, a tendency for electrons to collide with the solid surface is reduced due to the presence of the direct electric fields. This feature prolongs a lifetime of the electrons, resulting in generation of high-density plasmas (for example, at 100 MHz) inside the gas supply hole 25. The generated electric discharge is called hollow cathode discharge.
The hollow cathode discharge generated in the gas supply hole 25 causes deterioration of the gas supply hole (the lapse of time causes gradual increase of the diameter of the hole) 25 and contamination of a substrate by metal substances constituting the gas supply hole 25.
It is empirically indicated that a larger gas velocity in the gas supply hole 25 and a larger pressure gradient in the vicinity of the gas supply hole 25 tend to induce hollow cathode discharge. In addition, a larger gas flow rate and a lower pressure in the vacuum chamber 1 also tends to induce hollow cathode discharge. Accordingly, improvement of fine processability and implementation of a larger processing area require a larger flow rate of gas for use in processing and processing under lower pressure, which clarifies the importance of solving the issue of a hollow cathode discharge in the gas supply hole 25.